2. Field of the Invention
The present invention relates to an improvement of a universal joint, and more particularly to an improvement of a lubricant retaining mechanism for a universal joint including a journal cross.
2. Description of the Prior Art
A universal joint includes in general a journal cross having at its center an aperture for injecting a grease. The grease is fed through radially extending oil supply apertures along center lines of journal cross arms into oil reservoirs at the ends thereof for lubricating bearings thereat and roller bearings in bearing caps.
Although the grease has a reasonably high viscosity at a room temperature, it is completely liquid in the universal joint in operation because the joint is at a high temperature. When the universal joint operates at high speeds, its temperature generally rises considerably to as high as 200.degree. C. because of frictional heating, so that the grease in the universal joint is liquid devoid of any viscosity. Under such a condition, the liquid grease is forced toward the ends of the journal cross arms by centrifugal force to increase the pressure in the grease, with the result that the grease tends to leak from oil seals of the universal joint. In this manner, the decreased quantity of grease gives rise to various problems. Under these conditions, the universal joint of the prior art has mainly a following disadvantage.
When the rotation of the universal joint is stopped, the liquid grease in the journal cross arm that assumes an uppermost position falls down under the influence of gravity to leave a space that the grease previously had occupied and then the grease increases its viscosity as the temperature lowers. Accordingly, when the universal joint is moved again, the described space does not include any grease, so that the metal parts are brought into direct frictional contact with the other metal parts so as to cause abrasion or seizure of the parts.
Various solutions have been proposed to overcome the wear and defect resulting from the lack of oil. The assignee of this application has proposed a lubricant retaining device comprising a plunger in an oil supply aperture, adapted to be urged by a spring when the supplied oil pressure decreases to close the oil supply aperture, thereby preventing the lack of oil due to falling of the oil (Japanese Laid-Open Patent Application No. 78,820/80). Moreover, U.S. Pat. No. 3,470,711 issued on Oct. 7, 1969 discloses a lubrication system for a universal joint comprising an oil retaining element having a W-shaped cross-section in each journal cross arm of the joint to form one way valve at the center of the element, thereby preventing the lack of oil.
These devices of the prior art exhibit superior effects which solve the above problems to an extent. However, a simpler and more effective device is needed for the universal joint.